The present invention relates to an optical head for focusing a light beam for reading an information onto an information recording medium such as an optical disk, and more particularly, to an improvement in a photo detector for an optical head for detecting a defocusing state of a light beam.
Various optical information recording/reproduction systems have been recently devised which optically write and read information on and from an information recording medium (to be referred to as an optical disk hereinafter). For example, information recording/reproduction systems are known which are used for a recording medium for reproduction only such as a CD (compact disk) type of DAD or a video disk, an image file, a still image file, a COM (computer output memory) or the like. In such systems, information is recorded by establishing a state change such as forming a hole (pit) in a recording layer by a focused light beam. In other information recording/reproduction systems, information is optically written on or read from an information recording medium for recording/reproduction or from an erasable information recording medium (to be referred to as an optical disk hereinafter). In these systems, in both the write and read modes, a light beam must always be focused on the optical disk. In view of this requirement, the optical head frequently has a focus detecting system for detecting the state of focus of the light beam. Various such focus detecting systems have been proposed. For example, U.S. Pat. No. 4,521,680 issued June 4, 1985 and corresponding EPC application No. 82106508.3, filed on July 19, 1982 disclose a focus detecting system adopting the so-called knife edge method.
In this focus detecting system adopting the knife edge method, as shown in FIG. 1, a laser beam is reflected from a recording layer or light reflecting layer 10 on which information is to be or is already recorded. An objective lens 2, a knife edge 4, a convergent lens 6 and a photo detecting unit 8 are arranged on an optical path 0--0 of the reflected light from the recording layer 10. The knife edge 4 serves to allow passage of only that laser beam component which is spaced apart from the optical axis 0--0. The photo detecting unit 8 detects a laser beam which is converged by the convergent lens 6. The photo detecting unit 8 has a photo insensitive region 9 and two, first and second, photo sensing regions or photo sensitive regions 8-1 and 8-2 which is separated by the photo insensitive region 9, and is located at the back focal point of the convergent lens 6. In such a focus detecting system, the defocusing state is not detected by a change in the beam spot size on the photo detecting unit 8 but is detected by a deviation in the beam spot position in a direction 10. In the just in-focusing state, the beam spot is formed at the boundary between the two photo sensitive regions 8-1 and 8-2 of the photo detecting unit 8. A differential signal of the two photo signals from these regions 8-1 and 8-2 is kept substantially zero. In contrast to this, when the objective lens 2 draws too near to the recording layer 11 or is spaced too far apart therefrom, resulting in the defocusing state, the differential signal of the signals from the first and second photo sensitive regions 8-1 and 8-2 changes in the positive or negative direction.
In the focus detecting system described above, the photo sensitive regions 8-1 and 8-2 are separated by the photo insensitive region 9. However, when a width (distance between the photo sensitive regions 8-1 and 8-2) of the photo insensitive region 9 is excessively great, no defocusing signal will be generated from the photo detecting unit 8 even when the objective lens 2 is defocused and a beam spot is formed in the photo insensitive region 9 of the photo detecting unit 8. Conventionally, the width of the photo insensitive region 9 falls within a predetermined range, but sufficient consideration has not been given to the width of the photo insensitive region 9. In order to cause the photo detecting unit 8 to detect a defocusing state with satisfactory sensitivity, the width of the photo insensitive region 9 must fall within a predetermined range.